The basic biological community established under the prevailing environmental
conditions has the capacity to modify the sedimentary regime (biomodification). There are
several categories of biomodification:

by organisms with an ability to stabilise the sediment, (biostabilisation) as shown on
intertidal mud and sand flats, for example, by spionid tube beds (e.g. Prionospioelegans,
by affecting boundary conditions), microphytobenthic mats (by mucopolysaccharide
production), and eelgrass meadows (by sediment binding with rhizome production and by
disturbing the sediment-water interface turbulence);

by organism behaviour leading to biodestabilisation, which in turn may lead to increased
erosion (bioerosion); this may result from excessive reworking (bioturbation) by mobile
infaunal organisms (e.g. Macomabalthica) on mudflats;

by feeding behaviour increasing the supply of sediment from the water column to the
seabed through the production of faeces and pseudofaeces (biosedimentation), for example
by suspension feeders such as mussels (Mytilus edulis) on mudflats and cockles (Cerastoderma
edule) on sandflats.

Each of these processes modifies the sedimentary regime with the potential of
increasing its heterogeneity and thus the number of niches available for colonisation. For
example, extensive reworking increases the depth of surface-phenomena such as oxygenated
sediments as well as increasing rugosity (surface roughness). Surface roughness disrupts
the sediment-water boundary conditions and the ability for organisms to settle although it
may also increase erosion.

Heterotrophic marine organisms are predominantly deposit or suspension feeders. Deposit
feeders may feed at the surface or at depth within the sediment, resulting in the
production of faecal pellets and the movement of organic material from deeper within the
sediment to the surface. The vertical and lateral movement of mobile deposit feeders
causes the mixing and transport of particles, interstitial water and dissolved gases
(Rhoads, 1974). In muddier areas the production of faecal pellets by deposit feeders are
of a size which may be ingested or otherwise manipulated by other benthic invertebrates
hence increasing sediment reworking. As a consequence, the degree of bioturbation tends to
be greater in fine muds dominated by deposit-feeders than in coarse grained substrata
(Rhoads, 1974).

The factors most highly correlated with bioturbation are feeding method and location in
relation to the sediment-water interface, organism size and degree of mobility, population
density, burrowing depth and the density and spacing between animal tubes (Rhoads, 1982).
Many of these processes are population size and temperature dependent. In addition,
Reichelt (1991) identified three main processes leading to bioturbation: feeding activity,
burrow or tube construction and migration within the sediment column due to tidal and
diurnal cycles. For example, sedentary deposit-feeding polychaetes often form dense tube
aggregations which have a stabilising effect on the sediments. Suspension feeders actively
or passively entrap suspended seston which is later deposited at the sediment surface in
the form of faecal pellets or un-pelleted pseudofaeces. The upper size limit of particles
ingested by suspension feeders is generally smaller than that of deposit feeders
(JØrgensen, 1966 in Rhoads, 1974).

Faecal pellets have higher deposition rates than their constituent particles and
therefore settle out near the site of production. Deposit feeders may have a more
quantitatively significant role in pelletization of the sea floor than suspension feeders
or zooplankton (Rhoads, 1974). However, the production of non-pelleted pseudofaeces also
contributes to the rate of sedimentation in many mudflat areas.